In treating chronic renal failure, various methods of purification and treatment of blood with machinery are used for removing substances usually eliminated with the urine and for withdrawing fluids. Diffuse mass transport is predominant in hemodialysis (HD), but in hemofiltration (HF) convective mass transport through a membrane is used. Hemodiafiltration (HDF) is a combination of the two methods.
Because of the large exchange volumes, there is a need in these methods for accurate balancing of fluid withdrawn relative to fluid administered and the volume for ultrafiltration over the entire treatment time. Gravimetric and volumetric balancing systems are state of the art.
German Patent DE-PS 2,838,414 discloses a dialysis device having a volumetric balancing device. The balancing device consists of two chambers subdivided by a displaceable element and each having an inlet line for fresh dialysis fluid and an outlet line connected to a drain for spent dialysis fluid. Cutoff valves driven and switched by a control unit are arranged in the inlet and outlet lines. A pump is provided between the dialyzer and the balancing device to convey the spent dialysis fluid. In addition, a dialyzer valve is provided in the inlet line to the dialyzer, and an air separator is provided in the outlet line.
The known device is operated in such a way that fresh dialysis fluid is supplied from a dialysis fluid source to the two balancing chambers in alternation through appropriate switching of the cutoff valves in the inlet lines. At the same time, fresh dialysis fluid is supplied from an already filled space of the other balance chamber to the dialyzer, where the desired toxins are removed by diffusion from the blood which is also flowing through the dialyzer. The spent dialysis fluid is then pumped into the second space of the same balance chamber, from which the spent dialysis fluid then goes into an outlet.
The part of the liquid circuit enclosed between the balancing device and the dialyzer behaves like a closed, constant-volume system. A tapping device is provided to remove liquid from this system.
On the basis of the above-mentioned properties of the balancing device, the volume of fluid removed from the system by the tapping device is replaced by an equally large volume of fluid which goes from the blood side to the dialysis fluid side of the dialyzer membrane. The volume of fluid removed can be supplied to the patient again as replacement fluid. However, 1:1 replacement of the volume of fluid removed is not absolutely essential. Instead, it may be advantageous in individual cases to remove a larger volume of fluid than the volume of replacement fluid administered to the patient, i.e., despite replacement there may be a net ultrafiltration.
High demands are to be made on the accuracy of the tapping device, permitting control of ultrafiltration. To be able to withdraw a precisely predeterminable volume of ultrafiltrate from the dialysis fluid path, the tapping device is equipped with a volumetric diaphragm pump, with each individual pump stroke corresponding to a unit volume of ultrafiltrate. One disadvantage is that a separate pump with a relatively complicated design is necessary for removing a precisely predeterminable volume of ultrafiltrate.
German Patent DE 3,837,498 C2 describes a device where ultrafiltration is performed without an additional ultrafiltrate pump. The ultrafiltrate is not removed through a separate tapping device in the dialysis fluid path between the dialyzer outlet and the balancing device, but instead is removed in a controlled manner from the dialysis fluid path via the balancing device, with the dialysis fluid path being interrupted between two hemodialysis cycles. The balancing device of the known apparatus has two balance chambers. When ultrafiltrate flows into a first balance-chamber half of a first of the two balance chambers, fresh dialysis fluid from the second half of the first balance-chamber into either of a first or a second balance chamber half in the second balance chamber.
In addition, there is a device for ultrafiltration in dialysis, where the quantity of ultrafiltrate is determined by means of a balance chamber. The balance chamber is subdivided by a flexible partition into two chamber halves which are alternately filled with the ultrafiltrate tapped from the dialysis fluid path, with the contents of the other half of the chamber being diverted. However, it is impossible with the known device to balance ultrafiltrate against substituate.
A hemofiltration device where ultrafiltrate is balanced against substituate by means of a balance chamber is known from German Patent DE 4,116,178 C1.